A flight plan is the detailed description of the route to be followed by an aircraft within the framework of a planned flight. The flight plan is usually managed aboard civil aircraft by a system referred to as the “Flight Management System”, indicated as the FMS subsequently, which places the route to be followed at the disposal of the onboard personnel and at the disposal of the other onboard systems. This FMS system also allows an aid to navigation, through the display of information useful to the pilots, or else through the communication of flight guidance orders to an automatic piloting system.
FIG. 1 presents a summary diagram illustrating the structure of an FMSO known from the prior art. A known FMSO-type system has a man-machine interface MMI comprising for example a keyboard and a display screen, or else simply a touch-sensitive display screen, as well as at least the following functions, which are illustrated in a generic manner by an associated module and are described in the ARINC 702 standard:                Navigation LOC performs the optimal location of the aircraft as a function of the geo-location means GEOLOC such as satellite based geo-positioning or GPS, VHF radio navigation beacons, inertial units. This module communicates with the aforementioned geo-location devices. Thus the module LOC calculates the position (latitude, longitude, altitude) and the speed of the aircraft in space.        Flight plan FPLN inputs the geographical elements constituting the skeleton of the route to be followed, such as the points imposed by the departure and arrival procedures, the waypoints, the airways;        Navigation database NAVDB contains the waypoints, the geographical routes, the procedures and the beacons        Performance database PERFDB contains the craft's aerodynamic performance and engine parameters;        Lateral trajectory TRAJ, constructs by calculation a continuous trajectory on the basis of the points of the flight plan, using the performance of the aircraft and while complying with the confinement constraints (RNP);        Predictions PRED, constructs an optimized vertical profile on the lateral trajectory and provides the predictions in terms of time of transit, quantity of fuel remaining, altitude and speed of transit at each of the points of the flight plan.        Guidance GUID establishes, on the basis of the position and of the calculated trajectory, flight guidance orders so as to guide the aircraft in the lateral, vertical planes and speed flight targets so as to follow its three-dimensional trajectory, while optimizing its speed. The flight guidance orders are transmitted to the automatic pilot. When the aircraft is equipped with an automatic pilot PA and it is operative, it is this that transforms the flight guidance orders into flight control commands.        Digital data link DATALINK communicates with the air traffic control centres, the ground operational centres and, in the future, other aircraft 13.        
The flight plan is entered by the pilot, or else by data link, on the basis of data contained in the navigation database.
The pilot thereafter inputs the aircraft parameters: mass, flight plan, range of cruising levels, as well as one or a plurality of optimization criteria, such as the Cost Index CI. These inputs allow the modules TRAJ and PRED to calculate respectively the lateral trajectory and the vertical profile, that is to say the flight profile in terms of altitude and speed, which for example minimizes the optimization criterion.
Thus in a conventional manner a flight management system:                calculates a position of the aircraft (LOC) on the basis of data arising from onboard sensors listed hereinabove,        determines a trajectory (TRAJ/PRED module) with the databases PERF DB, in accordance with the flight plan defined on the basis of the NAV DB        provides, on the basis of the position and of the trajectory, flight guidance orders (GUID module), so as to follow this trajectory. In a conventional manner, the calculated aircraft position makes it possible to identify a possible deviation from the trajectory or a forthcoming change (turn, climb, acceleration or deceleration) of the trajectory. On the basis of this lateral deviation, GUID will establish a flight guidance order CG, in a conventional manner made up of: roll, heading or track laterally, pitch, speed, vertical speed, altitude or slope vertically, speed or thrust level in terms of speed. Hereinafter in the disclosure, the term “guidance order” covers the set of flight guidance orders as defined hereinabove.        
More precisely, the GUID module of the FMS computes the flight guidance orders on the basis of the position of the aircraft, of the part of the calculated trajectory that the aircraft is currently flying and of the guidance laws that are available in the automatic pilot and often specific to each aircraft. Among the laws of the automatic pilot, the FMS uses for example, laterally, the heading hold, vertically, the slope hold, the capture and hold of the altitude, in terms of speed the speed hold or thrust hold.
The trajectory calculated by the FMS comprises three components:                a lateral trajectory (latitude, longitude)        a vertical trajectory or profile        a speed profile—longitudinal axis        
The computed trajectory is always the same whatever the aircraft whereas the steering over this trajectory is dependent on the characteristics of each type of aircraft.
Guidance characterization is as follows: guidance along a lateral axis for the lateral guidance of the aircraft, along a vertical axis for the guidance in terms of altitude of the aircraft and along a longitudinal axis for the guidance in terms of speed.
The aircraft trajectory calculated by the FMS consists of an ordered series of segments that the aircraft follows as it progresses. Typically the segments are established according to the ARINC 424 standard legs of the flight plan. The current segment that the aircraft is presently flying is called the active segment.
The steering, that is to say the actual guidance of the aircraft, is performed on the active segment of the calculated trajectory. “Sequencing” refers to the identification of the active segment. This identification, carried out by the FMS, is essential for the generation of the flight guidance orders, associated with the active segment.
As illustrated in FIG. 2, the flight guidance orders generated by the GUID module of the FMS are transmitted to the automatic pilot PA. The PA transforms the flight guidance orders into flight controls CV applied directly to the aircraft symbolized by AC in FIG. 2. In a conventional manner, the automatic pilot generates and optionally dispatches to the control surfaces of the aircraft: the position (angle) for the ailerons and elevators, the thrust for the engines, etc.
Moreover, these flight controls are presented to the pilot via the flight director DV in the form for example of vertical and lateral bars (that the pilot must try to follow by hand when the automatic pilot is not engaged) on a display DISP.
An automatic pilot PA makes it possible to guide an aircraft automatically on the basis of flight guidance orders provided, either by the pilot (“tactical”) through an interface called FCU (AIRBUS) or MCP (BOEING) (so-called “selected” mode) or by a system of FMS type (“strategic” so-called “managed” mode). We are interested here in guidance on the basis of the FMS. In a conventional manner, the automatic pilot determines the deviation between the current attitude (roll, pitch) of the aircraft and the desired flight guidance order (pilot selection or guidance control of the FM) and generates a flight control command CV on the basis of a piloting law.
An automatic pilot operates according to various modes, depending on the distribution of the guidance between the PA and the pilot.
In the manual mode, the pilot guides the aircraft laterally and vertically by providing flight controls with his stick and guides the aircraft in terms of speed by providing thrust controls with the throttle.
The PA is said not coupled. When the PA is coupled it is said “engaged”.
In the so-called “selected” mode the pilot delegates guidance of the aircraft to the automatic pilot and to the auto throttle. He selects flight guidance orders (heading, altitude, slope, speed) through a control panel that the automatic pilot transforms into flight controls for the lateral and the vertical, and into thrust for the speed.
In the “managed” mode, termed “lateral and vertical and speed managed”, also called “full managed”, guidance is carried out laterally on the basis of the lateral trajectory, vertically on the basis of the vertical profile in terms of latitude, and in terms of speed, on the basis of the speed profile. The pilot delegates to the FM the selection of the flight guidance orders to follow the flight plan. The FM automatically selects the flight guidance orders for following the flight plan and dispatches them to the automatic pilot and to the auto-throttle which transforms them into flight and thrust controls.
According to a “lateral managed, vertical selected, speed selected” mode the managed guidance of the PA is a lateral guidance solely, on the basis of the lateral trajectory, the pilot remaining in charge of the controls for the altitude and the speed of the aircraft.
According to the prior art there exist various types of automatic pilots which are more or less elaborate: certain automatic pilots accept only roll and pitch inputs from the FMS. Others in addition to these basic inputs, afford access to higher flight targets such as heading, altitude, slope. Yet others PA are responsible for all of the steering laws, including that the FMS usually uses to provide roll and pitch, the FMS then providing only deviations with respect to the reference trajectory.
Thus, today, most FMSs are designed to prepare and steer an aircraft over a complete reference trajectory (“managed” mode see above) automatically. Unfortunately, though the complete trajectory (also called a 4D profile) from one aircraft to another is still characterized by the succession of rectilinear or curved segments, the steering over this trajectory is eminently different therefrom. Consequently, current flight management systems must still modify their guidance function to take account not only of the specifics of the aircraft's performance, but also of the aircraft manufacturer's philosophy characterized by the interface between the flight management system and the automatic pilot system and therefore of the scope that he grants to each of these systems.
The problem which comes therefrom is that, each time that the FMS is developed for a new aircraft or for a new aircraft manufacturer, it is necessary not only to modify the guidance function to adapt it to its new environment, but also to fully recertify the FMS since this function is internal to this system.
Certain procedures require a more significant level of precision in the guidance of the aircraft. For example, towards the end of the cruising phase and a few minutes before beginning the descent, the pilot selects via the FMS the approach procedure that he will use to put the aircraft down on the landing runway of his destination airport. The approach procedure for certain airports is of the RNP AR type with RNP<0.3 NM.
The RNP concept used in the aeronautical industry consists on the one hand in the capacity of the aircraft's navigation system to monitor its performance (precision) and to inform the pilot as to whether or not the operational requirements (error) are adhered to during the operation, and on the other hand in the optimization of the approach procedures by basing them on the aircraft's navigation performance. This concept makes it possible to reduce the spacings between aircraft while cruising and in the terminal zone, to optimize the takeoff and landing procedures. It also makes it possible to reduce the minima associated with the approach procedures both in non-precision approaches and in conventional RNAV approaches.
An RNP procedure refers to a specific procedure or a block of space. For example, an RNP procedure xx signifies that the navigation systems of the aircraft must be capable of calculating the position of the aircraft in a circle of xx Nm, for example an RNP 0.3 in a circle of 0.3 Nm.
Approaches of RNP AR type require continuity and integrity of the guidance even after a simple fault. Apart from a problem inherent to the conventional FMS system is that the calculation of a trajectory flyable by the aircraft (in accordance with its performance) and of precise predictions use complex algorithmic calculations, and that this complexity is a source of fault called reset. These faults cause the loss of the FMS, and in the current state not only is the trajectory lost but in addition the guidance on this trajectory is lost since both these functions are managed by the lone FMS system.
Thus, this type of approach requires FMS architectures which render the guidance function which steers the aircraft over the reference trajectory more robust and available. Availability of the trajectory is particularly significant in the procedures with RNP AR with RNP<0.3 nm.
In the prior art, there exists a solution for reducing the loss and increasing the integrity of the guidance function. This solution relies on an architecture with 3 FMSs and therefore exhibits a much higher cost, with the purchase of a 3rd instance of FMS, and higher electrical consumption, because of the electrical consumption of this additional item of equipment.
An aim of the present invention is to alleviate the aforementioned drawbacks by proposing a method and a system for the guidance of an aircraft making it possible to carry out this guidance independent of the functional core of the FMS.